Thanks for the response Gez.

I read the manual wrong. I was trying to use white to add to my Quick mask when 
I should have been using black. There are TWO types of masks: a LAYER mask and 
a CHANNEL mask (aka SELECTION mask). The QUICK mask is a SELECTION mask used to 
refine a selection. When you toggle the Quick mask Off, the transparent areas 
become a “selection”.

“A Quick Mask is a Selection Mask intended to be used to temporarily paint a 
selection. Temporarily means that, unlike a normal selection mask, it will be 
deleted from the channel list after its transformation to selection.”  GIMP 
Manual

LAYER Masks operate this way:
White ADDS to mask
Black REMOVES the mask
Grey makes mask SEMI-TRANSPARENT

CHANNEL masks (and Quick/Selection masks) operate the opposite way:
White REMOVES the mask
Black ADDS to mask
Grey makes mask SEMI-TRANSPARENT

Regards,
Rick



-----Original Message----- 
From: Gez 
Sent: Saturday, May 14, 2016 12:42 AM 
To: gimp-user-list@gnome.org 
Subject: Re: [Gimp-user] Adding to Quick Mask using a white brush 

El jue, 05-05-2016 a las 14:15 -0400, Rick Strong escribió:
> I’m trying to add to a Quick Mask using white as both foreground and
> background colours. White is supposed to add to the mask. But it
> keeps removing the mask. In other words, using a white brush deletes
> that part of the mask and reveals the image underneath when it should
> be covering it up. v. 2.8.16
> 
> Any ideas?

It looks like you got it wrong.

Think about black and white as 0 and 1 respectively.
In a selection, 0 means "it's not selected" and 1 means "it's
selected".

This is also consistent with layer masks, where black makes pixels
transparent while white makes them opaque.
Since you can produce layer masks from selections, it makes sense that
those values are consistent.

And why are black and white used that way in layer masks? Because masks
work like alpha channels.

So, when you paint white, you're painting the pixels you want to be selected, 
the ones to be visible.
It makes sense that those are not "masked out" by your quick mask.

Gez.
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